Citrus fruits are known to be rich in antioxidants, such as eriocitrin, 6,8-di-C-glucosyldiosmetin, and 6-C-glucosyldiosmetin, belonging to polyphenols. These antioxidants derived from natural materials can be utilized favorably in various industrial products such as food or drink products, pharmaceuticals, and cosmetics. These antioxidants can be extracted from citrus fruits according to previously known processes described in Patent Documents 1 to 3.
Patent Documents 1 and 2 respectively describe a process for producing a polyphenol-containing material, which comprises purifying, using reverse-phase resin treatment or liquid chromatography, an extract obtained from a citrus material using water, an organic solvent, or a mixed solvent thereof.
Patent Document 3 describes a process for producing a food material containing eriocitrin at a high concentration. The process described in Patent Document 3 comprises: obtaining an extract from at least one of juices, peels, and squeezed residue of citrus fruits using an extractant; applying the extract to a porous synthetic adsorption resin such that eriocitrin is adsorbed on this resin; for the removal of impurities, washing, with water, the synthetic adsorption resin comprising the eriocitrin adsorbed thereon; and then separating and recovering the eriocitrin adsorbed on the synthetic adsorption resin using an organic solvent.
The production processes described in Patent Documents 1 and 2 involve fractionation based on high-performance liquid chromatography for enhancing polyphenol purity in the obtained polyphenol-containing material. However, this procedure, which is complicated and time-consuming, results in increase in the cost of producing the polyphenol-containing material. Alternatively, the production process described in Patent Document 3 involves, for the removal of impurities, washing, with water, the synthetic adsorption resin comprising the eriocitrin adsorbed thereon. However, foods supplemented with the obtained eriocitrin-containing material, particularly, foods supplemented with this material at a high concentration, still have bitterness or other unpleasant tastes and leave unpleasant aftertaste in the mouth after being ingested. The eriocitrin-containing material produced by the process described in Patent Document 3 has a peculiar, Chinese herbal medicine-like smell (crude drug smell) attributed to heat applied in extraction, concentration, and purification steps. In addition, general polyphenol-containing materials tend to have offensive smells or browning due to changes over time. Thus, their long-term quality maintenance is difficult to attain.    Patent Document 1: Japanese Laid-Open Patent Publication No. 9-48969    Patent Document 2: Japanese Laid-Open Patent Publication No. 10-245552    Patent Document 3: Japanese Laid-Open Patent Publication No. 2000-217560